a) Field of the Invention
The present invention relates to a process for preparing high purity methacrylamide suitable for use in modifiers in the field of windshield glasses, photographic films and coagulating agents by purification of an aqueous crude methacrylamide solution.
b) Description of the Prior Art
Many processes have been known for the preparation of methacrylamide.
These processes are, for example, a process for reacting acetone cyanhydrin (hereinafter referred to as ACH) with sulfuric acid and neutralizing the resulting methacrylamide sulfate to obtain methacrylamide; and a process for hydrating methacrylonitrile to obtain methacrylamide.
The methacrylamide prepared by the above processes are satisfactory for use in the field of fiber modifiers and the raw material for preparing emulsion of coating and adhesive.
The above methacrylamide, however, cannot be used in view of quality for windshield glasses and other applications. High degree of transparency is required for many resin products used for these applications, and hence contamination of a trace amount of impurity such as polymer leads to serious deterioration of quality in these products.
Purification steps are required for preparing the high purity methacrylamide and recrystallization is the most practical process of purification. As a recrystallization process, for example, Japanese Patent 50666 (1984) (U.S. Pat. No. 4,465,856) discloses crystallization under alkaline conditions.
However, in the purification of methacrylamide by recrystallization, the mother liquor obtained by separating the crystal is preferably recycled as a raw material to the next crystallization step. Repeated recycling leads to a problem of rapid deterioration in the quality of methacrylamide.
In order to inhibit deterioration of quality, the mother liquor is often treated with activated carbon. The treatment, however, cannot maintain satisfactory quality of methacrylamide and a portion of the mother liquor must be discharged out of the system.